The present invention relates to an optical element using in an image projection apparatus which includes a reflective image-forming element.
A so-called three-plate type reflective projector performs color separation of illumination light of a blue wavelength range, a green wavelength range, and a red wavelength range, and color combination of image light modulated by each of three reflective image-forming elements. Elements such as a dichroic element and a polarization beam splitter are therefore arranged between the reflective image-forming element and the projection lens.
In a known general three-plate type reflective projector, between one reflective image-forming element and a projection lens, at least two elements are provided which perform color separation or color combination and have a splitting film forming an angle of 45° with respect to the optical axis of an illumination optical system. In other words, in the projector, three or four elements including a dichroic element and a polarization beam splitter are arranged.
Such a projector using three reflective liquid crystal panels respectively corresponding to a blue wavelength light, a green wavelength light and a red wavelength light and performing color separation and color combination by the dichroic element and the polarization beam splitter has been disclosed in Japanese Patent Laid-Open No. 2001-154152.
Moreover, in Japanese Patent Laid-Open No. 2002-162520, a prism type element which performs color separation, polarization beam splitting, and color combination in one element has been disclosed. This prism type element has a configuration in which three wavelength-selective polarization splitting films are arranged in a cube. In the cube, three surfaces each of which forms an angle of 45° with respect to two surfaces contacting one of three sides gathering to one apex and includes the one side, are formed. In other words, three 45° surfaces passing one apex are formed. And desired wavelength-selective polarization splitting films are arranged along these 45° surfaces.
On the other hand, as an optical thin film such as a polarization splitting film, a multilayer film is used in many cases. Moreover, it is also known that utilizing the fact that P-polarized light is transmitted therethrough at Brewster's angle can provide a polarization splitting film with a wide polarization beam splitting wavelength range.
In ‘Li Li and J. A. Dobrowolski, Appl. Opt., vol. 39, p. 2754, 2000’ a polarization beam splitter which reflects P-polarized light and transmits S-polarized light has been disclosed. In the polarization beam splitter, light impinges at a large incident angle that is larger than a critical angle obtained from the refractive index of a prism and a low refractive-index thin film. In usual, although total reflection occurs and light is not transmitted through the thin film, since the film thickness of the low refractive-index thin film is equal to or smaller than the wavelength of the light, attenuated total reflection occurs and thereby light is transmitted through the thin film. Utilizing the attenuated total reflection can provide a polarization beam splitter which reflects P-polarized light, transmits S-polarized light, and has a wider incident angle range, a wider polarization beam splitting wavelength range and a higher extinction ratio, as compared to those of a common polarization beam splitter using Brewster's angle.
Moreover, in Japanese Patent Laid-open Nos. 2006-47903 and 2006-79058, a wavelength-selective polarization splitting film has been disclosed, which transmits S-polarized light and reflects P-polarized light in a specific wavelength range, and reflects S-polarized light and transmits P-polarized light in another specific wavelength range. In these documents, a multilayer polarization splitting film having a wavelength-selective function is obtained by utilizing attenuated total reflection and using a specific film configuration.
In a projector disclosed in Japanese Patent Laid-Open No. 2001-154152, first, illumination light is color-separated into light of two wavelength ranges (first and second wavelength lights) and light with one wavelength light (third wavelength light). After that, the first wavelength light and the second wavelength light are color-separated and color-combined, and finally, the first wavelength light, the second wavelength light and the third wavelength light are color-combined. Therefore, two elements for performing color separation or color combination are required in the optical path of one wavelength light. Therefore, in the whole projector, four elements are required, and thereby the configuration thereof becomes complicated. Moreover, the optical path length thereof becomes long, thus, in order to secure required amount of the luminous flux, each of the elements becomes also large. Furthermore, since the back focus of a projection lens becomes long, the projection lens itself becomes also large.
Moreover, in a prism type element disclosed in Japanese Patent Laid-Open No. 2002-162520, the first wavelength light, the second wavelength light and the third wavelength light are color-separated and color-combined by one element. However, there is a boundary line due to a beam splitting film in each of all optical path surfaces in the prism type element. Among them, a boundary line existing in an optical surface for a reflective liquid crystal panel largely influences images, and thereby each wavelength light from each of the panels becomes light being substantially in parallel with respect to another wavelength-selective polarization splitting film other than two wavelength-selective polarization splitting films acting as beam splitting films. Since the incident angle with respect to the wavelength-selective polarization splitting film of the light is large, the reflectance thereof for the light becomes high. As a result, a ghost is likely to occur.
On the other hand, a conventional optical thin film has a characteristic in which, depending on the incident angle of light, a light-transmission action appears strongly or a reflection action appears strongly. For example, in the polarization beam splitter using Brewster's angle, if the incident angle of P-polarized light deviates from the conditions of Brewster's angle, reflection will occur. Therefore, it has been difficult to obtain the light-transmission action by an incident angle different from Brewster's angle at which polarization beam splitting is performed.
The polarization beam splitter disclosed in ‘Li Li and J. A. Dobrowolski, Appl. Opt., vol. 39, p. 2754, 2000’ also does not have a function of causing light components in the entire visible wavelength range impinging on its polarization beam splitting surface at different incident angles to be transmitted therethrough.
Moreover, even the wavelength-selective polarization splitting films disclosed in Japanese Patent Laid-open Nos. 2006-47903 and 2006-79058, do not have a function of causing light components impinging thereon at different incident angles to be transmitted therethrough.